Kidneys regulate the volume of blood by monitoring and maintaining what gets retained and what gets excreted via chemical make-up of blood. This way, the kidneys (1 on each side) keep the balance of salts, water, acids, bases, glucose, etc. for our whole body.

Kidneys will also get rid of toxins, excess water, ions, and normal metabolic waste from our body in the form of urea (normal breakdown of amino acids).

Functional unit of the kidney = Nephron.

This is one of the maaaaaany nephrons that make up each kidney

The PCT (proximal convoluted tubule) reabsorbs things like glucose, amino acids, proteins, vitamins, etc.
The Loop of Henle is for reabsorbing has 2 parts: a Descending part and an Ascending part

The Descending limb of the loop of Henle is for reabsorbing water. This creates a concentration gradient where urea will be absorbed into the loop to be sent to the Collecting Duct.

The Ascending limb of the loop of Henle now reabsorbs ions (sodium & chloride ions) to balance out the concentration.

The DCT (distal convoluted tubule) reabsorbs sodium, bicarbonate, water, and chlorine ions, etc.

The Collecting Duct is regulated by hormones aldosterone and vasopressin (antidiuretic hormone) to reabsorb and excrete electrolytes and fluids (sodium, potassium, bicarbonate, water). Fluids in the collecting duct eventually drain into the papillary duct to go to deliver urine to the minor calyx. Eventually, the minor calyx goes to the major calyx, then renal pelvis, then ureter to go to the bladder to be stored for excretion, where then it will leave the body through the ureter.